Electric vehicles are becoming commercially important. Such vehicles include a traction battery and a traction motor. Some current proposals use a hybrid concept, in which an additional power source, such as a motor/generator, is used to improve the range of the vehicle by recharging the traction battery during operation. Since the vehicle is powered by a direct-voltage traction battery, some such vehicles use direct-voltage motors. The direct-current motor has brushes, which are a wear point. The dc brushless motor is a permanent-magnet motor, which is energized by alternating voltages. The induction motor is more robust, and may be cheaper to build in relatively high power output than the dc brushless motors.
Induction require variable-voltage, variable frequency alternating voltages for complete control. Some induction-motor controllers use Voltage/frequency (V/F) controllers, but the control is less thorough than Field-Oriented Control (FOC), and it is difficult to cause the induction motor to rotate above the "nominal" motor speed, which is the no-load speed at which the motor will spin when the specified drive voltage is applied. The induction motor can typically be spun at rotational speeds much greater than the nominal. It is desirable to connect induction traction motor to the drive wheels by a fixed gear ratio, in order to avoid the cost and weight of a transmission. When the traction motor is directly connected to the drive wheels, the considerations which determine the fixed gear ratio may be understood by considering as an example an induction motor which has a nominal speed of 4000 RPM, and which has a maximum possible (breakdown) rotational speed of 16000 RPM. If, on the one hand, it were desired to limit the maximum motor speed to 4000 RPM, and the desired maximum vehicle speed were to be 65 MPH, the gear reduction ratio would be four times less than if the maximum speed had been chosen as 16,000 RPM. Put another way, the gear reduction ratio which gives 65 MPH at 16000 RPM is greater than that required for 65 MPH at 4000 RPM. This larger gear reduction ratio, in turn, advantageously produces greater torque over the entire speed range. Therefore, the induction motor should be operated near the nominal bus voltage.
FIG. 1 is a simplified plot 1 of the motor speed .omega. versus field strength .lambda. characteristics of an induction motor, In FIG. 1, plot 1 includes a portion 2 which is constant at a flux value of .lambda..sub.nominal from zero to the nominal motor speed .omega..sub.nominal. This portion of the plot represents an operating region in which the motor is not limited by the applied bus voltage, but rather by other considerations such as maximum winding current. Region 3 of plot 1 is a region in the motor speed is higher than the nominal speed .omega..sub.nominal, in which the back electromotive force is given by EQU V.sub.EMF =.omega..multidot..lambda..sub.nom 1
which indicates that at the nominal speed, the back EMF equals the specified bus voltage. The motor speed designated .omega..sub.max represents the speed at which centrifugal forces are expected to cause the rotor to disintegrate, and therefore represents an absolute maximum upper speed limit. At speeds .omega. higher than nominal speed .omega..sub.nominal, the back EMF exceeds the bus voltage, so long as the flux field is maintained. Thus, the field strength must be reduced in order to operate at speeds above the nominal, in order to reduce the back EMF, so that some voltage differential remains between the bus voltage and the back EMF to cause motor current. However, reducing flux also reduces torque. In order to maintain the maximum amount of torque over the range of motor speeds from nominal speed to the selected maximum allowable speed (less than .omega..sub.max, the back EMF must be maintained just below the bus voltage. Thus, the motor always operates near the limit of bus voltage in the region above the nominal speed. As a result, the stability may be compromised during those periods in which the controller demands application of more motor voltage than the maximum bus voltage.
State-limited proportional-integral regulators are known for use in control circuits, as described, for example, in "Nonlinear Algorithms for Fast and Robust Control of Electrical Drive" by Dusan Borojevic, PHD Dissertation, 1986, Virginia Polytechnic Institute & State University, Blacksburg, Va. FIG. 2 is a simplified block diagram of a control system 10 according to the prior art. In FIG. 2, reference signal or user's command is applied by way of a terminal 12 to a noninverting (+) input port 14i1 of an error signal generator 14. It should be understood that the signals described herein represent the values which their names suggest, so that, for example, a motor field voltage command signal represents, whether directly or by a proportionality constant, the actual value of field, or of the field current, or of the corresponding values of the field itself or of the current producing the field as measured in other reference systems, so that the signal processing can be described in terms of the representative signals or the values which the signals represent. Error signal generator 14 of FIG. 2 subtracts from the reference signal a feedback signal representing the controlled variable which is applied to inverting (-) input port 14i2, to produce the system error signal. The error signal is applied to first and second multipliers 16 and 18, respectively. First multiplier 16 multiplies the error signal by a constant K.sub.p, as known in the art, to produce a proportional command component at its output port 16o. The proportional command component is applied to an input port 20i of a limiter 20. Limiter 20 limits the range of the proportional command to lie between maximum and minimum values applied to its upper limit port 20ul and lower limit port 20ll, respectively. The upper limit signal or value applied to upper limit port 20ul of limiter 20 is produced by MAX CMD source 22, and the lower limit signal or value applied to lower limit input port 20ll is produced by MIN CMD source 24. Limiter 20 produces a limited proportional command signal at its output port 20o.
Second multiplier 18 of FIG. 2 multiplies, by a constant K.sub.i, the error signal applied to its input port 18i, to thereby produce a signal at its output port 18o which, due to the action of the feedback loop as described below, represents the first derivative of the integral component of the limited integrated command. State-limited integrator 26 integrates the signal from output port 18o of multiplier 18, to produce an integrated signal which is limited by limiting the integration state of the limiter 26. More particularly, the integration state of state-limited integrator 26 is limited to lie below an upper value applied to its upper limit port 26ul, and to lie above a lower value applied to its lower limit port 26ll. The upper limit signal applied to the upper limit port 26ul of state-limited integrator 26 is represented by the output signal of a subtracting or differencing circuit 28. Differencing circuit 28 takes the difference between the limited proportional component of the command, from output port 20o of limiter 20, and the MAX CMD value from source 22. Similarly, the lower limit signal applied to the lower limit port 26ll of state-limited integrator 26 is produced by a differencing circuit 30, which subtracts the limited proportional component of the command signal from the MIN CMD value produced by source 24. Thus, the upper integration limit of integrator 26 is limited to lie below the maximum limit established by differencing circuit 28, and the lower integration limit of integrator 26 is limited to lie above the minimum limit established by differencing circuit 30. The output at output port 260 of limiting integrator 26 is the limited integrated component of the command signal.
A summing circuit 32 of FIG. 2 sums the limited proportional components and the limited integrated component of the command signals, to produce the total command signal for controlling the plant 34. The plant 34 responds to the command signals to adjust the controlled variable and therefore the feedback signals which return to the inverting input port 14i2 of error signal generator 14 to close the degenerative feedback loop. The arrangement of FIG. 2 has the advantage of, in the presence of a transient input or reference signal, limiting overshoot attributable to slow response of the plant in conjunction with the time constant of the integrator.
When two control limiters were used to control the field and torque components of an induction motor, the motor control was unstable in certain operating modes. Improved induction motor controllers are desired.